Method of monitoring pressure of a gas species and apparatus to do so

ABSTRACT

A method of monitoring pressure of a gas species up to at most a predetermined maximum pressure value is disclosed. The method includes exposing the gas species to transmission of laser light, periodically modulating the wavelength of the laser light over a wavelength band including at least one absorption line of the gas species, optoelectrically converting the transmitted laser light, thereby generating an electric output signal, performing at least one of first filtering the electric output signal with a filter characteristic having a lower cut-off frequency not lower than a transition frequency and of second filtering the electric output signal with a bandpass filter characteristic having an upper cut-off frequency not higher than the transition frequency and a lower cut-off frequency above the modulation frequency of the periodic wavelength modulation. The output of at least one of the filterings is evaluated as a pressure indicative signal.

The present invention is directed on a novel method and apparatus and onvarious embodiments thereof for monitoring pressure of a gas species. Itresulted from the need of rapidly monitoring oxygen content intransparent closed containers, as of glass or plastic material vials,e.g. for medical appliances.

Under a first aspect a first embodiment of the method according to thepresent invention of monitoring of a gas species up to at most apredetermined maximum pressure value, such method comprises

-   -   exposing the gas species to transmission of laser light;    -   periodically modulating the wavelength of the laser light over a        wavelength band which includes at least one absorption line of        the gas species;    -   optoelectrically converting the transmitted laser light, thereby        generating an electric output

and it further comprises at least one of

-   -   -   first filtering the electric output signal of converting            with a filter characteristic which has a lower cut-off            frequency which is not lower than a transition frequency and            of        -   second filtering the electric output signal of the            conversion with a band pass filter characteristic which has            an upper cut-off frequency which is not higher than the            transition frequency and with a lower cut-off frequency            above modulation frequency of the periodic wavelength            modulation.

    -   Thereby, the transition frequency in the spectrum of the        electric output signal of conversion is determined there where        the caustic function of the pressure-dependent spectrum        envelopes of the addressed output electric signal touches the        envelope of the spectrum at the maximum pressure.

    -   The output of the addressed at least one filtering is evaluated        as a pressure indicative signal.

In one embodiment both, namely first and second filtering, areperformed.

In one embodiment the first filtering is performed as band passfiltering.

In one embodiment the first filtering is performed with a lower cut-offfrequency which is higher than the addressed transition frequency.

In another embodiment the first filtering is performed as band passfiltering and there is determined a filter frequency above the lowercut-off frequency of the addressed first filtering there where thederivatives of spectral amplitude vs. pressure of the electric outputsignal of the conversion at least approximately accord with a desiredcharacteristic. Then band pass first filtering is performed with thedetermined filter frequency as band pass center frequency.

In a further embodiment the first filtering is performed as band passfiltering and bandwidth of this band pass filtering is selected with thetarget of achieving a desired signal to noise ratio.

Still in a further embodiment the first filtering is performed as a bandpass filtering and a desired sensitivity of the output signal of theband pass first filtering is realized under consideration of noise inthat the followings steps are performed once or more than one time in alooping manner:

-   (a) A filter frequency is determined above the lower cut-off    frequency of the first filtering there where the derivative of    spectral amplitude vs. pressure of the electric output signal of    conversion at least approximately accords with a desired    characteristic. Band pass center frequency of the addressed band    pass first filtering is established at the filter frequency thus    determined.-   (b) The bandwidth of the band pass first filtering is designed for a    desired signal to noise ratio.

In a further embodiment the method according to the present inventioncomprises selecting the upper cut-off frequency of the second filteringbelow the addressed transition frequency.

Still in a further embodiment of the method according to the presentinvention the second filtering is performed with a center frequencythere where the derivative of spectral amplitude vs. pressure of theelectric output signal at least approximately accords with a desiredcharacteristic.

Still in a further embodiment the second filtering is performed with abandwidth selected for a desired signal to noise ratio.

In a further embodiment the second filtering is performed, therebyrealizing a desired sensitivity of an output signal of the addressedsecond filtering under consideration of noise by performing thefollowing steps subsequently once or in a looping manner more than onetime:

-   (a) Determining a center frequency of the second filtering there    where the derivative of spectral amplitude vs. pressure of the    electric output signal of conversion at least approximately accords    with a desired characteristic and-   (b) tailoring bandwidth of the second filtering for a desired signal    to noise ratio.

Under a second aspect of the present invention there is established themethod of monitoring pressure of a gas species within a predeterminedpressure range, namely between a maximum pressure value and a minimumpressure value. This latter method comprises

-   -   exposing the gas species to transmission of laser light;    -   periodically modulating the wavelength of the laser light over a        wavelength band which includes at least one absorption line of        the gas species;    -   optoelectrically converting the transmitted laser light, thereby        generating an electric output signal;    -   then at least one of the following steps is performed        -   first filtering of the electric output signal of conversion            with a filter characteristic which has a lower cut-off            frequency not lower than a transition frequency and        -   second filtering of the electric output signal of conversion            with a band pass filter characteristic which has an upper            cut-off frequency which is not higher than the transition            frequency and with a lower cut-off frequency which is above            modulation frequency of the periodic wavelength modulation.    -   Thereby, the addressed transition frequency in this case is        determined in the spectrum of the electric output signal there        where the spectrum envelopes of the electric output signal at        the minimum and at the maximum pressure values intersect.    -   Then the output signal of at least one of the addressed first        and second filtering is evaluated as a pressure indicative        signal.

In one embodiment both the first and second filtering is performed

-   -   under the second aspect.

In a further embodiment under the second aspect the first filtering isperformed as band pass filtering.

Under another mode of the second aspect of the present invention thefirst filtering is performed with a lower cut-off frequency which ishigher than the transition frequency.

Under the second aspect another embodiment comprises performing thefirst filtering as band pass filtering between the transition frequencyand a noise limit frequency. The noise limit frequency is therebydefined there where noise energy of the electric output signal ofconversion equals signal energy of that electric output signal at thepredetermined minimum pressure value.

A further embodiment under the second aspect comprises selecting theband pass first filtering so that the energy difference in the filteredspectrum of the electric output signal between the maximum pressureapplied and the minimum pressure applied becomes maximal.

Still a further embodiment under this second aspect comprises selectingband pass first filtering under the constraint that noise energy of theelectric output signal, there where the filtering is effective, becomesat most equal to signal energy at the predetermined maximum pressurevalue.

Still in a further embodiment under this second aspect the upper cut-offfrequency of the second filtering is selected below the transitionfrequency.

Still another embodiment under this second aspect comprises performingthe second filtering there where the energy difference in the spectrumof the electric output signal of conversion is maximum between applyingthe maximum pressure value and the minimum pressure value which bothestablish for the pressure range to be monitored under this secondaspect of the invention.

Under a third aspect the present invention provides for a method formonitoring pressure of a gas species which comprises

-   -   exposing the gas species to transmission of laser light;    -   periodically modulating the wavelength of the laser light over a        wavelength band which includes at least one absorption line of        the gas species;    -   optoelectrically converting the transmitted laser light, thereby        generating an electric output signal;    -   inputting a signal which is dependent on the electric output        signal of conversion to at least a first and a second parallel        gas pressure monitoring channel;    -   performing in the first channel first filtering and in the        second channel second filtering;    -   performing the first filtering so that the output signal thereof        varies with a first characteristic as a function of pressure of        said gas species;    -   performing the second filtering so that the output signal        thereof varies with a second characteristic as a function of the        addressed gas pressure;    -   further performing said first and second filtering so that the        first characteristic becomes different from the second        characteristic.    -   From combining signals which are dependent on the output signals        of the first and of the second filtering a pressure indicative        signal is evaluated.

In an embodiment of this third aspect at least one of the first and ofthe second filtering is performed as band pass filtering.

In an embodiment under this third aspect first and second filtering isperformed in non-overlapping frequency areas of the spectrum of theelectric output signal.

In a further embodiment under this third aspect the first and the secondfiltering are performed as band pass filtering.

In a further embodiment under the third aspect of the present inventionthe first and second filtering are performed in respective first andsecond frequency ranges, whereby the energy of the electric outputsignal has a first energy vs. pressure characteristic in the firstfrequency range and has a second energy vs. pressure characteristic inthe second frequency range, whereby the first and second energycharacteristics are different from each others.

In a further embodiment which is applicable to the present inventionunder all its aspects there is prestored a first referencecharacteristic which represents the first characteristic and/or there isprestored a second reference characteristic which represents the secondcharacteristic. The addressed characteristics are the characteristicswith which the output signal of the respective filtering varies as afunction of pressure of the gas species to be monitored.

Then momentary signals which depend from signals of the first and/orsecond filterings respectively are compared with a stored first andsecond reference characteristic, whereby first and second pressureindicative signals are generated. Thus, momentarily prevailing filteringresults become compared with predetermined signal vs. pressurecharacteristics to establish from the prevailing signals to whichpressure value they accord.

Still in another embodiment under the third aspect of the presentinvention, but also applicable to the invention under the first andsecond aspects, the first filtering generates a first output signalwhich has first derivatives vs. pressure within a predetermined pressurerange. The second filtering generates a second output signal which hassecond derivatives vs. pressure in the addressed predetermined pressurerange. Thereby, absolute values of one of the addressed first and secondderivatives are smaller than absolute values of the other of theaddressed derivatives in at least one common subpressure range of thepredetermined pressure range. Thereby, when speaking of signalderivatives we clearly disregard signal noise which makes derivativevalues random.

In a further embodiment under the third aspect but also applicable tothe invention under the first and second aspects the second filteringgenerates a second output signal which has exclusively positive ornegative derivatives vs. pressure in a predetermined pressure range.

The first filtering generates a first output signal with firstderivatives vs. pressure which are exclusively positive in at least onepressure subrange of the predetermined pressure range and which areexclusively negative in at least one second subrange of the addressedpredetermined pressure range. Thereby, the absolute values of the secondderivatives are smaller in at least one of the addressed pressuresubranges than the absolute values of the first derivatives alsoconsidered in the addressed at least one of the subranges and againwithout considering noise.

In an embodiment especially under the third aspect of the presentinvention from a signal dependent on the second output signal of secondfiltering one of the pressure subranges is determined.

In a further embodiment from this pressure subrange determined and fromthe first output signal, i.e. of first filtering, a pressure indicativesignal is determined.

Under the third aspect of the present invention a further embodimentcomprises

-   -   predetermining a maximum pressure to be monitored;    -   performing at least one of the following steps, namely        -   performing the second filtering with a band pass filter            characteristic which has an upper cut-off frequency not            higher than a transition frequency and which has a lower            cut-off frequency which is above modulation frequency of            periodic wavelength modulation and        -   performing first filtering with a filter characteristic            which has a lower cut-off frequency which is not lower than            the transition frequency.    -   Thereby, the addressed transition frequency is determined in the        spectrum of the electric output signal there where the caustic        function of the pressure-dependent spectrum envelopes of the        electric output signal touches the envelope of the spectrum at        the maximum pressure.

Thereby, in one embodiment first as well as second filtering areperformed.

Thereby, in a further embodiment first filtering is performed as bandpass filtering.

Thereby, in a further embodiment first filtering is performed with alower cut-off frequency higher than the transition frequency.

Thereby, in another embodiment first filtering is performed as a bandpass filtering and there is determined a filter frequency which is abovethe lower cut-off frequency of first filtering there where thederivative of spectral amplitude vs. pressure of the electric outputsignal of conversion at least approximately accords with the desiredcharacteristic. Then band pass first filtering is performed with thedetermined filter frequency as band pass center frequency.

Thereby, another embodiment comprises performing the first filtering asband pass filtering, thereby selecting bandwidth of band pass firstfiltering with the target of achieving a desired signal to noise ratio.

Thereby, a further embodiment comprises selecting an upper cut-offfrequency of the second filtering below the transition frequency.

Thereby, a further embodiment comprises performing the second filteringwith a center frequency there where the derivative of spectral amplitudevs. pressure of the electric output signal at least approximatelyaccords with a desired characteristic.

Thereby, still a further embodiment comprises performing the secondfiltering with a bandwidth for a desired signal to noise ratio.

Thereby, an embodiment further comprises performing the second filteringand realizing a desired sensitivity of the output signal of the secondfiltering under consideration of noise by performing the following stepssubsequently once or repeatedly in one or more than one loops:

-   a) Determining a center frequency of the second filtering there    where the derivative of spectral amplitude vs. pressure of the    electric output signal at least approximately accords with a desired    characteristic and-   b) tailoring bandwidth of the second filtering for a desired signal    to noise ratio.

Still under the third aspect one embodiment comprises

-   -   performing monitoring in a predetermined pressure range between        a minimum pressure value and a maximum pressure value;    -   performing at least one of the following steps:        -   second filtering with a band pass filter characteristic            which has an upper cut-off frequency not higher than a            transition frequency and which has a lower cut-off frequency            which is above modulation frequency of the periodic            wavelength modulation;        -   first filtering with a filter characteristic which has a            lower cut-off frequency not lower than the transition            frequency;    -   whereby the transition frequency is determined in the spectrum        of the electric output signal there where the spectrum envelopes        of the electric output signal at said minimum pressure value and        at said maximum pressure value intersect.

Thereby, a further embodiment comprises performing both first and secondfilterings. Still a further embodiment comprises performing the firstfiltering as a band pass filtering.

Thereby, a further embodiment comprises performing the first filteringwith a lower cut-off frequency which is higher than the just addressedtransition frequency. Thereby, in another embodiment first filtering isperformed between the transition frequency and a noise limit frequency.The noise limit frequency is defined there where noise energy of theelectric output signal equals signal energy of the electric outputsignal at the addressed minimum pressure value.

Thereby, in a further embodiment band pass first filtering is selectedso that the energy difference in the filtered spectrum of the electricoutput signal between applying the maximum pressure value and theminimum pressure value becomes maximum.

Thereby, still a further embodiment comprises selecting the band passfirst filtering under the constraint that noise energy of the electricoutput signal there where the first filtering is effective is at mostequal to signal energy at the maximum pressure as predetermined.

Thereby, in a further embodiment the upper cut-off frequency of thesecond filtering is selected below the transition frequency asaddressed.

Still further, in an embodiment second filtering is performed therewhere the energy difference in the spectrum of the electric outputsignal of conversion is maximum between applying the maximumpredetermined pressure value and the minimum predetermined pressurevalue.

Still in a further embodiment under all three aspects of the presentinvention the gas species monitored is oxygen.

Still in a further embodiment under all three aspects of the presentinvention there is generated a reference pressure indicative signal byperforming the monitoring according to the present invention at apredetermined pressure of the gas species. Yet under a furtherembodiment there is generated a resulting pressure indicative signal independency of a difference of the reference pressure indicative signaland the pressure indicative signal.

Still a further embodiment under all the aspects of the presentinvention comprises monitoring transmission of the laser light along atrajectory path to which the gas species is or is to be applied andthereby generating a transmission indicative signal. Signals from whichthe pressure indicative signal depends are then weighted in dependencyof the transmission indicative signal.

Still a further embodiment of the invention under all its aspectsfurther comprises providing a gas with the gas species in at least onecontainer which is transparent for the addressed laser light.

Thereby, in a further embodiment such container is provided andmonitored in ambient air.

Still in a further embodiment monitoring transparency of the trajectorypath for the laser light includes the addressed transparent container,whereby a transparency indicative signal is generated. Signals wherefromthe pressure indicative signal depends are then weighted in dependencyof the transmission indicative signal.

Thereby, in a further embodiment there is provided a third parallelchannel as a calibration channel and there is generated the addressedtransmission indicative signal in the said calibration channel.

In a further embodiment a reference signal is generated by performingthe monitoring according to the present invention at a referencecontainer with a predetermined amount of the gas species, whereby aresult pressure indicative signal is generated in dependency of thedifference from said reference signal and the pressure indicativesignal.

Still a further embodiment comprises checking the pressure indicativesignal on plausibility.

Still a further embodiment comprises monitoring oxygen pressure in theaddressed containers.

Still a further embodiment comprises that the addressed containers arefilled with a product.

Still in a further embodiment the addressed containers are substantiallyof glass or plastic material. Thereby, still in a further embodimentsuch containers are vials.

Still in a further embodiment a container, whereat the gaseous contentis monitored, is one of a multitude of containers which are conveyed ina stream towards, past and from the addressed monitoring.

Still in a further embodiment the container being monitored and thelaser light transmitted therethrough are moved in synchronism duringmonitoring.

Still a further embodiment comprises monitoring subsequently the gaspressure in a multitude of containers which are conveyed subsequentlytowards, past and from the monitoring, thereby generating a referencepressure indicative signal by applying to the monitoring at least onereference container with a predetermined amount of the gas species to bemonitored before monitoring one of the multitude of containers. There isthereby generated a result pressure indicative signal in dependency ofthe reference pressure indicative signal and the pressure indicativesignal.

Further, in an embodiment the reference pressure indicative signal isgenerated each time just before monitoring one of the multitude ofcontainers.

Still in a further embodiment reference pressure indicative signals fromsubsequent monitoring of reference containers with the predeterminedamount of gas species are averaged and the addressed difference isformed in dependency of the result of such averaging.

The present invention is further directed on a method for manufacturingclosed, possibly filled containers which are transparent to laser lightand with a predetermined maximum amount of oxygen, which manufacturingmethod comprises the steps of manufacturing closed, possibly filled andtransparent containers and subjecting these containers to a gas pressuremonitoring as was disclosed above and according to one of the differentaspects and embodiments of the present invention and rejectingcontainers if a signal which depends from the pressure indicative signalis indicative for an oxygen pressure in such container above apredetermined maximum value.

There is further proposed according to the present invention anapparatus for performing the present invention under all its methodaspects.

With an eye especially on monitoring oxygen pressure the followingdimensioning rules may be followed:

Center frequency of second filtering band pass, f_(ZII), relative tolaser wavelength modulation frequency f_(C):10≦f _(ZII) /f _(C)≦20

Center frequency of band pass applied for first filtering, f_(ZI)relative to modulation frequency f_(C):50≦f _(ZI) /f _(C)≦120

Pass bandwidth of second filtering relative to modulation frequency:1≦B _(II) /f _(C)≦18

Pass bandwidth of band pass first filtering, B_(I) relative tomodulation frequency f_(C):50≦B _(I) /f _(C)≦1000

Deviation H of wavelength modulation of the laser:

-   -   At least 5 pm, thereby preferably        50 pm≦H≦500 pm.

Further, one laser which may be applied in context with the presentinvention is the Vertical Cavity Surface Emitting laser.

The invention shall now be described by way of examples and with thehelp of figures. The figures show:

FIG. 1 simplified and schematically, an apparatus according to thepresent invention and performing the monitoring method according to thepresent invention;

FIG. 2 qualitatively over the wavelength axis, absorption lines of a gasspecies to explain wavelength modulation according to the presentinvention;

FIG. 3 the absorption spectra of a gas species as of oxygen at differentpressures;

FIG. 4 qualitatively the spectrum of electric signals resulting fromoptoelectric conversion according to the present invention;

FIG. 5 qualitatively envelopes of spectra according to FIG. 4 atdifferent pressures of gas species;

FIG. 6 qualitatively envelopes of the spectra according to FIG. 4 fordifferent gas pressures defining for the caustic function;

FIG. 7 a representation in analogy to that of FIG. 6 for explainingdetermination of transition frequency with the help of the causticfunction;

FIG. 8 schematically and qualitatively first filtering according to thepresent invention in one frequency area relative to the transitionfrequency as found according to FIG. 7;

FIG. 9 a simplified functional block diagram showing evaluation offiltering results according to FIG. 8;

FIG. 10 qualitatively an example of the course of an output signal ofthe embodiment of FIG. 9 as a function of pressure to be monitored;

FIG. 11 schematically by means of a signal flow/functional block diagramevaluation of a pressure indicative signal according to the presentinvention and with filtering according to FIG. 9;

FIG. 12 schematically and qualitatively second filtering according tothe present invention in a second frequency area with respect totransition frequency determined according to FIG. 7;

FIG. 13 simplified, a functional block representation of performingfiltering according to FIG. 12;

FIG. 14 qualitatively an example of the dependency of the output signalof the embodiment according to FIG. 13 from the pressure to bemonitored;

FIG. 15 simplified and schematically by means of a signalflow/functional block diagram evaluating, a pressure indicative signalfrom the filtering according to FIG. 13;

FIG. 16 a further embodiment of the present invention, whereby filteringaccording to FIG. 8 and filtering according to FIG. 12 are exploited incombination;

FIG. 17 qualitatively, spectral envelopes of the spectra according toFIG. 4 for different pressure values for explaining tailoring the bandpass filter according to FIG. 8;

FIG. 18 in a representation according to FIG. 17, tailoring band passfilter according to FIG. 12;

FIG. 19 again parameterized with the pressure, qualitatively spectralenvelopes of spectra according to FIG. 4 under a second aspect of theinvention for determining a transition frequency;

FIG. 20 in analogy to FIG. 8, tailoring first filtering in a frequencyarea above transition frequency as determined in FIG. 19;

FIG. 21 a representation in analogy to FIG. 9, thereby filteringaccording to FIG. 20;

FIG. 22 qualitatively an example of the dependency of output signal ofembodiment according to FIG. 21 from gas pressure;

FIG. 23 in analogy to FIG. 11 evaluation of the result of filteringaccording to FIG. 21 for generating a pressure indicative signal;

FIG. 24 in analogy to FIG. 12 a representation of second filtering in afrequency area below transition frequency determined according to FIG.19;

FIG. 25 in analogy to FIG. 13 a simplified functional block diagram forperforming filtering according to FIG. 24;

FIG. 26 in analogy to FIG. 15, in a simplified form and by a functionalblock/signal flow diagram evaluating filtering according to FIG. 25 forgenerating a pressure indicative signal;

FIG. 27 qualitatively, an example of the dependency of output signal ofthe embodiment of FIG. 25 from pressure;

FIG. 28 a further embodiment according to the present invention andbased on transition frequency as determined according to FIG. 19,thereby combining filtering according to FIGS. 20 and 24 for evaluatinga pressure indicative signal;

FIG. 29 schematically and simplified, a twin-parallel measuring channelembodiment according to the present invention;

FIG. 30 the result of monitoring oxygen pressure with the embodimentaccording to FIG. 29 in one pressure range;

FIG. 31 in a representation according to FIG. 30, the results in asecond pressure range;

FIG. 32 in a representation according to the FIGS. 30 and 31, theresults when monitoring over a larger pressure range;

FIG. 33 a functional block/signal flow diagram of a part of anevaluation unit according to the present invention exploiting signalcourses as e.g. shown in FIG. 32;

FIG. 34 most simplified, an apparatus according to the present inventionand operating according to the method of the present invention fortesting transparent closed containers in a stream with respect to gascontent;

FIG. 35 a simplified signal flow/functional block diagram of anevaluation unit in a further variant, and

FIG. 36 a variant according to the present invention of the apparatusaccording to FIG. 34.

In FIG. 1 there is shown by means of a functional block diagram thegeneric structure of a monitoring system according to the presentinvention for monitoring pressure of a gas. The system comprises a laserarrangement 1 generating a laser beam B transmitted through a specimen 3of gas containing the gas species to be pressure monitored. The laserbeam B as transmitted through the specimen 3 is received at an opticalinput of an optoelectric converter arrangement 5 and is converted in anelectric signal S₅ which is operationally connected to the input E₇ ofan evaluation unit 7. The output signal S₇ at output A₇ of evaluationunit 7 is a pressure indicative signal indicative of the pressure of thegas species in specimen 3.

The laser arrangement 1 is modulatable with respect to λ_(L) of thelight of beam B. As schematically shown in FIG. 1 it may be consideredhaving a wavelength control input M to which there is operationallyconnected a modulating generator 9. Generator 9 generates a periodicmodulating signal to input M of laser arrangement 1 at a frequency f_(C)and with a peak to peak level value A_(pp). Thereby, the beam B havinggenerically a wavelength λ_(L) is wavelength-modulated with a frequencyf_(C) around the value λ_(Lo) with a modulation hub H as a function ofA_(pp).

In FIG. 2 there is qualitatively shown absorption line E_(abs)(G) of agas species G to be monitored. The laser arrangement 1 is therebywavelength-modulated so that the modulation hub H includes at least oneof the absorption lines of the gas species, according to FIG. 2 e.g.just one absorption line.

In fact, the absorption line as schematically shown in FIG. 2 is anabsorption spectrum as shown in FIG. 3. This fig. shows as an examplefor a gas species to be pressure monitored, namely for oxygen, theabsorption spectra for a pressure of 200 mbar (a), 75 mbar (b) and 40mbar (c) of the specimen exclusively containing the gas species.

The absorption spectrum and its pressure dependency as exemplified foroxygen in FIG. 3 is with respect to its qualitative shape and behaviorencountered for most gas species.

By modulating the wavelength of the laser beam B according to FIG. 1 andas shown in FIG. 2 and by transmitting the laser beam B through the gasspecimen 3 as of FIG. 1 having the absorption spectrum as shown in FIG.3, converted signal S₅ has a discrete energy spectrum as shownqualitatively in FIG. 4. There is a normally relatively high spectralline at zero frequency, as a DC component succeeded at rising frequencyby a spectral line at modulation frequency f_(C) of the laser beam andhigher order frequencies of f_(C). The discrete energy spectrum ofsignal S₅ defines for the spectrum envelope EN as shown for the exampleof gas specimen, namely oxygen, qualitatively in FIG. 4.

In FIG. 5 the pressure—p—dependency of the envelope EN, thus EN(p) isshown, whereby the envelope according to (a) accords with the 200 mbarabsorption spectrum (a) of FIG. 3 and, accordingly, the envelopes (b)and (c) with the respective absorption spectra (b) and (c) of FIG. 3. Ithas to be noted that the spectra and respectively their envelopes EN(p)as shown in FIGS. 4 and 5 are shown with a logarithmic energy scaling onthe vertical axis e.g. according to a logarithm of signal energy tonoise energy in dB.

In FIG. 6 there is purely qualitatively shown a multitude of envelopesEN(p) parameterized by the pressure p of the gas species in specimen 3,whereby the arrow “p_(increase)” indicates the development of theenvelopes as pressure p of the gas species increases. The collectivityof all the envelopes EN defines for a function course CA to which eachof the collectivity of envelopes EN is a tangent. We call this functionCA the caustic function.

The inventors of the present invention have investigated the behavior ofsignal S₅ in dependency of the pressure p of the gas species in specimen3 of FIG. 1 and have found the spectral behavior in dependency ofpressure as has been exemplified up to now here without a claim forscientific exactness.

This basic cognition has been exploited by the inventors in differentmanners as will be now further exemplified with the target to generateat the output of evaluation unit 7 as of FIG. 1 a pressure indicativesignal S₇.

According to a first aspect of the present invention and as shown inFIG. 7 the inventors have recognized that whenever the pressure of thegas species is to be monitored up to maximum pressure p_(max) atransition frequency f_(Tmax) is important. If according to FIG. 6 themaximum pressure p_(max) up to which the pressure p of the gas speciesshall be monitored is selected, now switching to FIG. 7, the locus wherethe envelope EN(p_(max)) touches the caustic function CA in the spectralrepresentation defines for the addressed transition frequency f_(Tmax).In both frequency areas, I above f_(Tmax) and II below T_(max) thepressure dependency of S₅ according to FIG. 1 is specific. Thereby, ithas to be noted that the modulation frequency f_(C) is practicallyalways below f_(Tmax), especially for selected maximum pressures p_(max)as performed in practice. Thus, in a first embodiment the spectrum of S₅is evaluated in the first addressed frequency area I to generate apressure indicative signal.

It has to be noted that we speak generically of a frequency “area” if aone-side open frequency range is addressed and that we speak of afrequency “band” if a two-side closed frequency range is addressed.

According to FIG. 8 in one embodiment there is thus only evaluated thefrequency area I, i.e. with a frequency f_(I)F_(Tmax)≦f_(I)e.g. by providing a high-pass filter with the lower cut-off frequencynot lower than the transition frequency f_(Tmax).

Thus, the evaluation unit 7 as of FIG. 1 at least comprises, accordingto FIG. 9, a filter unit F_(I) operating in the frequency area I as alow-pass filter with a lower cut-off frequency not lower than thetransition frequency f_(Tmax). An example of the pressure—p—dependencyof the output signal S_(FI) of filter F_(I) up to the maximum pressureto be monitored p_(max), is shown in FIG. 10, qualitatively. Energy andpressure are thereby linearly scaled in arbitrary units.

By prestoring a signal vs. pressure characteristic, as shown in FIG. 11denoted as S_(FIref) in a memory unit 11 _(I), as e.g. in a look-uptable unit, comparing the actual output signal S_(FI)a of filter unitF_(I) with such characteristic in a comparing unit 13 _(I), the actualpressure value p_(a) as shown in dashed lines in FIG. 10 is evaluated asa pressure indicative signal. This signal may directly be used as outputsignal S₇ of evaluation unit 7.

In a second embodiment of the invention and with an eye on FIG. 7, thelower frequency area II, which is actually a frequency band, isexploited. Thereby, filtering is performed as shown in FIG. 12 by afilter with an upper cut-off frequency not higher than the transitionfrequency f_(Tmax) and with a lower cut-off frequency which is above themodulation frequency f_(C) of the laser wavelength modulation. Thus,there is performed in this embodiment as a second possible filtering,band pass filtering with a characteristic having an upper cut-offfrequency not higher than the transition frequency and a lower cut-offfrequency above the modulation frequency f_(C) of the periodicwavelength modulation.

In this embodiment and according to FIG. 13 the evaluation unit 7comprises a filter unit F_(II) providing for the filter characteristicaccording to FIG. 12, thereby generating an output signal S_(FII) independency of which the pressure indicative signal S₇ is generated. Heretoo, signal S_(FII) is already are per se pressure indicative.

In analogy to FIG. 10, FIG. 14 shows a qualitative dependency of outputsignal S_(FII) from the pressure p of the gas species in specimen 3 ofFIG. 1 to be monitored.

In complete analogy to FIG. 11 and according to FIG. 15 for generating asignal which is indicative of the actual pressure of the gas specimen,the characteristic S_(FIIref) of output signal as of FIG. 14 is storedin a storing unit 11 _(II), e.g. in form of a look-up table. Thischaracteristic S_(FIIref) is compared, as schematically shown bycomparing unit 13 _(II), with the actual signal S_(FIIa), therebygenerating at the output of comparing unit 13 _(II) a signal accordingto the actual pressure value p_(a) in dependency of which the pressureindicative signal S₇ is generated.

In a further embodiment, the embodiments according to the FIGS. 8 to 11and of FIGS. 12 to 15 are combined. This is shown in FIG. 16 which onlyneeds little additional explanation in view of the explanations given tothe respective single-filter embodiment. At the output of the respectivecomparing units 13 _(I) and 13 _(II) there appear signals which are bothin fact indicative of the actually prevailing gas species pressure p_(a)in the gas specimen being monitored. Nevertheless, and as addressed bythe different shaping of the characteristics in FIGS. 10 and 14, theredundancy of having two output signals indicative of the same actualpressure value p_(a) is exploited in a computing unit 15, principally torise accuracy of pressure indication by the pressure indicative signalS₇.

Generically spoken as evaluation of both frequency areas I and II byaccording filtering leads to two different signal dependencies frompressure according to S_(FI) and S_(FII)—these two characteristics maybe different with respect e.g. to sensitivity (steepness of thecharacteristic), ambiguity, signal to noise ratio, etc.—it becomespossible to remedy the drawback of one characteristic, e.g. ambiguity,by the advantage of the second characteristic, e.g. unambiguity, therebymaintaining the advantage of the first characteristic, e.g. highsensitivity, without making use of the drawbacks of the secondcharacteristic, e.g. low sensitivity. Thus, principally by exploitingfiltering measurements in the two frequency areas—above and below thetransition frequency f_(Tmax)—high flexibility is gained to generate anaccurate pressure indicative signal with desired characteristics, whichwill normally be high sensitivity, unambiguity and high signal to noiseratio.

Further and with an eye on filtering in the first frequency area Iaccording to FIG. 8 and as shown in that figure with dashed-pointedlines, it is a variant to perform this first filtering as band passfiltering. The reason why this is considered may be seen in FIG. 6. Asthe envelope EN according to the selected maximum pressure p_(max)crosses the zero line as at point N of FIG. 6, noise energy becomespredominant. Therefore, by performing band pass filtering in frequencyarea I, noise energy in the signal S_(FI) is reduced.

Further, and especially with an eye on combining the filtering in thefirst frequency area I with filtering in the second area II according tothe embodiment of FIG. 16, separation of the two filters is reached byperforming first filtering F_(I) with a lower cut-off frequency f_(I−)higher than the transition frequency f_(Tmax) as shown in FIG. 8 and/orto perform the second filtering F_(II) as shown in dashed line in FIG.12 with an upper cut-off frequency f_(II+) at a frequency below thetransition frequency f_(Tmax).

Whenever first filtering comprises band pass filtering in a frequencyband BP_(I) of FIG. 8, there remains still a degree of freedom, wherethe center frequency f_(ZI) of this band pass filtering shall beestablished.

FIG. 17 shows a representation according to FIG. 6. When considering inthe spectral representation in frequency area I, the energy in signalS_(FI) at a frequency f is in fact given, in dependency of pressure p,by the infinitesimal surface area as shown in hatched representation inFIG. 17. Therefore, it might be seen that at a prevailing pressure pthis energy is proportional to the prevailing spectral amplitude atfrequency f, with respect to envelope EN(p). Sensitivity is therebygiven by the derivative of that spectral amplitude vs. pressure. It hasfurther to be noted (logarithmic scale) that whenever apressure-dependent spectral envelope EN(p) reaches zero dB line, thenoise energy in signal S₅ becomes equal to the signal energy therein,i.e. below that zero dB line, the noise energy becomes predominant.Therefore, it is one rule to find the central frequency f_(ZI) of bandpass filtering in frequency area I, to establish a desiredcharacteristic of derivative of spectral amplitude vs. pressure and toselect as the addressed central frequency f_(ZI) of FIG. 8 thatfrequency f where this derivative accords at least approximately withthe desired characteristic. Thereby, it has to be noted that a maximumderivative vs. pressure leads to maximum sensitivity. Because thecharacteristic of spectral amplitude vs. pressure at a given frequency fis not linear, one may e.g. select where, i.e. in which pressure range,maximum sensitivity shall be reached, in other words in which pressurerange maximum sensitivity shall be realized.

Thereby, signal to noise ratio may be an additional target value. Is isdependent on the bandwidth BP_(I) of band pass filtering in frequencyarea I. The larger the bandwidth BP_(I) is selected, the smaller willbe, as a generic rule, sensitivity in that the derivative of signalenergy vs. pressure will be decreased, but, on the other hand, thelarger will be the signal to noise ratio. This becomes evident whenagain considering FIG. 17, where a filter frequency band BP_(I) has beendrawn in dashed line. The derivative of spectral energy of the signalS_(FI) is represented by the spectral surface area hatched under BP_(I),the derivative of which vs. pressure becoming smaller with increasingbandwidth BP_(I), the noise energy represented by the surface area underBP_(I) below zero dB becoming relatively smaller with increasingbandwidth.

Therefore, one approach to select central frequency f_(ZI) underconsideration of signal to noise ratio is to loop one or more than onetime through the steps of

-   a) determining the central band pass filtering frequency f_(ZI) by    finding that frequency f at which the derivative of spectral    amplitude vs. pressure accords best with the desired characteristic;-   b) selecting bandwidth BP_(l) with respect to the central frequency    found additionally considering signal to noise ratio and    sensitivity, possibly readjusting center frequency and bandwidth    with the target of realizing a desired optimum compromise between    sensitivity and signal to noise ratio.

We have now described how filtering in frequency area I may be realized.Let's turn now to considerations about filtering in frequency area II.

As was explained in context with FIG. 12, filtering in frequency rangeII is performed by band pass filtering with an upper cut-off frequencyf_(II+) at most at f_(Tmax) and with a lower cut-off frequency f_(II−)above modulation frequency f_(C) of laser wavelength modulation.

According to FIG. 18 and with an eye on selecting central frequency ofband pass filtering, the same considerations prevail as were explainedin context with FIG. 17 for band pass filtering in the frequency area I.That is, and with an eye on FIGS. 18 and 12, the central frequencyF_(ZII) is selected by finding the frequency f whereat the derivative ofspectral amplitude vs. pressure at least approximately accords with adesired characteristic. Thereby, additionally and with respect toselecting frequency band BP_(II) of band pass filtering F_(II) signal tonoise ratio is considered, although being less critical as becomesevident from FIG. 18 than in the frequency area I. Again, withsensitivity to be reached as one target function and signal to noiseratio as a second target function the steps of

-   a) determining the center frequency f_(ZII) of filter F_(II) is    performed by establishing where the derivative of spectral amplitude    vs. pressure of the signal S_(FII) (FIG. 13) at least approximately    accords with a desired characteristic and-   b) tailoring bandwidth BP_(II) for a desired signal to noise ratio    are performed once or more than one time in a looping manner.

We have now described the present invention under the aspect of havingan upper maximum pressure p_(max) established up to which the pressureof the gas species shall be monitored.

Let's now consider a further aspect, where there is established apressure range between a maximum pressure p_(max) and a minimum pressurep_(min) in which the pressure of the gas species shall be monitoredaccording to the invention.

Thereby, FIGS. 1 to 5 still prevail as well as the explanations whichwere given in context with these figures.

FIG. 19 shows a representation in analogy to FIG. 6. It shows thespectral envelope for the maximum pressure to be monitored EN(p_(max))as well as the spectral envelope for minimum pressure to be monitoredEN(p_(min)). Thus, the pressure range Δp inclusive the limit values atp_(max) and p_(min) shall be monitored. In this case there is determineda transition frequency f_(TΔp) there, where the envelope EN(p_(min))crosses the spectral envelope EN(p_(max)). In analogy to f_(Tmax) thistransition frequency f_(TΔp) delimitates two frequency areas I_(Δp) andII_(Δp).

In one embodiment and according to FIG. 20 there is performed in thefrequency area I_(Δp) filtering as of high-pass filtering with a lowercut-off frequency f_(ΔpI−) which is not lower than the transitionfrequency f_(TΔp). This results in an embodiment of the presentinvention according to FIG. 21 having a filter F_(ΔpI) operating in thefrequency area I_(Δp) of FIG. 19 as a high pass filter. The pressureindicative signal S₇ of evaluation unit 7 depends on output signal offilter F_(ΔpI), S_(FΔpI). There results as a qualitative example anoutput signal S_(FΔpI) of filter F_(ΔpI), according to FIG. 22. Energyand pressure are thereby again linearly scaled in arbitrary units.

In FIG. 23 there is shown in analogy with FIG. 11 the structure of theevaluation unit 7 for evaluating the filter signal S_(FΔpI) to generatea pressure indicative signal S₇. In view of the explanations given incontext with FIG. 11, FIG. 23 and the respective embodiment becomesperfectly clear to the skilled artisan and needs no further explanation.

In a further embodiment of the invention and with an eye on FIG. 19 thelower frequency area II_(Δp), which is again actually a frequency band,is exploited. Thereby, filtering is performed as shown in FIG. 24 by afilter with an upper cut-off frequency S_(ΔpII+) not higher than thetransition frequency f_(TΔp) and with a lower cut-off frequencyS_(ΔpII−) which is above the modulation frequency f_(C) of the laserwavelength modulation.

In this embodiment and according to FIG. 25 the evaluation unitcomprises in analogy to the embodiment as shown in FIG. 13 a filter unitF_(ΔpII) providing for the filter characteristic according to FIG. 24,thereby generating an output signal S_(FΔpII), in dependency of whichthe pressure indicative signal S₇ is generated. Again the signalS_(FΔpII) is per se pressure indicative.

In analogy to FIG. 15, FIG. 26 shows the structure of the evaluationunit 7 exploiting the output signal S_(FΔpII) of filter F_(ΔpII) toestablish for the pressure indicative signal S₇. Here too, no additionalexplanations are necessary for the skilled artisan. Further, FIG. 27shows in analogy to FIG. 14 quantitatively an example of the dependencyof output signal S_(FΔpII) from the pressure p of the gas species inspecimen 3 of FIG. 1 being between the minimum and maximum pressuresp_(min) and p_(max).

In FIG. 28 a further embodiment is shown combining the embodiment asexemplified and explained with the help of the FIGS. 20 to 23 with theembodiment as explained in context with the FIGS. 24 to 27. The targetof combining these embodiments is the same as was discussed in contextwith the combined embodiment of FIG. 16. In view of FIG. 16 and theexplanations given thereto the embodiment of FIG. 28 needs no additionalexplanations for the skilled artisan.

With an eye on FIG. 8 and the explanations given thereto also in thecase of providing filtering in the frequency area I_(Δp) according toFIG. 20, and as shown in dashed-dotted lines, band pass filtering isapplied. Further, again based on the explanations given in context withFIG. 8, in one embodiment, especially in the embodiment making use offiltering in both frequency areas I_(Δp) and II_(Δp), the lower cut-offfrequency f_(ΔpI−) of filtering in the frequency ares I_(Δp) is higherthan the transition frequency f_(TΔp). Further, and referring to FIG. 12and the explanations thereto, in a further variant according to FIG. 24the upper cut-off frequency f_(ΔpII+) is selected below the transitionfrequency f_(TΔp).

In spite of the fact that, with an eye on FIGS. 19 and 20, band passfiltering F_(FpI) in frequency area I_(Δp) may be performed with respectto selecting central frequency f_(ZΔpI) and bandwidth, underconsiderations similar to those given in context with FIGS. 17 and 18,it is of prime importance here that such band filtering is establishedbetween the transition frequency f_(TΔp) and a noise limit frequencyf_(N) (see FIG. 19) where at p_(min) the noise energy in signal S₅equals signal energy therein. Thereby, such band pass filtering may beperformed so that the energy difference in the spectrum between theenvelope EN(p_(max)) and the envelope EN(p_(min)) becomes maximum.

It has to be noted that, as was mentioned, the spectral envelopes EN(p)as shown in the various figures are merely qualitative. Nevertheless, inFIG. 19 a bandwidth BP_(ΔpI) is arbitrarily shown with the hatchedsurface representing the addressed energy difference.

Principally band pass filtering in frequency area I_(Δp) is performed byrespective selection of center frequency f_(ZΔpI) (see FIG. 20) andbandwidth BP_(ΔpI) under the constraint that noise energy of the outputsignal S₅ where the filtering is effective is at most equal to signalenergy at maximum pressure to be evaluated.

Considering frequency area II_(Δp) and with an eye on FIG. 24 band passfiltering is performed with a lower cut-off frequency f_(ΔpII−) abovemodulation frequency f_(e) of the periodic wavelength modulation andwith an upper cut-off frequency f_(ΔpII+) which is at most equal to thetransition frequency f_(TΔp). Again with an eye on FIG. 19 the centralfrequency and the bandwidth of this filtering is selected so as toperform band pass filtering there, where the energy difference in theenvelope spectrum of signal S₅ between applying the maximum pressure tobe monitored p_(max) and the minimum pressure p_(min) to be monitored tobecome maximum. The respective energy difference to be considered isshown in FIG. 19 in frequency area f_(IIΔp) in hatched representation.

We have shown embodiments for evaluating pressure indicative signalsmaking use of single filtering above or below of a transition frequencyf_(Tmax) and f_(TΔp) respectively or to combine filtering in twoparallel channels, thereby performing filtering in both frequency areasabove and below the respective transition frequency f_(Tmax), f_(TΔp).Thus, in the latter embodiment and as shown in FIG. 29 at least two,namely a first monitoring channel K_(I) and a second monitoring channelK_(II), are formed with filtering F_(I) and F_(II), respectivelytailored in dependency on how transition frequency f_(T) has beendefined. Thus, FIG. 29 is an embodiment as of FIG. 16 or 28.

Thus, dependent on the output signal of the respective first and secondfiltering F_(I), F_(II) in the respective channels K_(I) and K_(II) thepressure indicative signal is computed.

Thereby, there is exploited that the result of the respective first andsecond filtering in the channels K_(I) and K_(II) becomes different.I.e. the output of filter F_(I) varies as a function of the pressure ofthe gas species to be monitored differently than the result of thesecond filtering F_(II).

In FIG. 30 there is shown the result of monitoring pressure of a gasspecimen, thereby of oxygen as an example, by showing the output signalS_(FI) and S_(FI) of filtering in the respective channels K_(I) andK_(II) as of FIG. 29. The signals S_(FI,II) are scaled in arbitrary butequal units.

For filtering in frequency area I a band pass filter is used. Thefilters are not optimized with respect to sensitivity and signal tonoise ratio. The lower cut-off frequency of band pass filter in channelK_(I) as well as upper cut-off frequency of band pass filter in channelK_(II) are nevertheless selected so as to be clearly distant from bothtransitions frequencies f_(Tmax) and f_(TΔp).

It may be seen that there is generated a first output signal S_(FI) offirst filtering which has in the pressure subrange Δp_(S) of about 0mbar to 50 mbar out of the overall monitored pressure range Δp of about0 mbar to 75 mbar a derivative vs. pressure, the absolute value of whichbeing larger than the absolute value of the derivative vs. pressurecharacteristic of the signal S_(FII), i.e. of filtering in channelK_(II). In FIG. 31 there is shown in a representation analog to that ofFIG. 30 the respective output signals S_(FI) and S_(FII) of therespective channel filtering in a different range of pressure, again asan example of oxygen pressure, namely in the range of about 75 to 200mbar. From FIG. 31 it might be seen that here within the pressuresubrange Δp_(s) of about 110 to 190 mbar the absolute value of thederivative of signal S_(FI) vs. pressure is larger than such derivativeof the signal S_(FII). Thus, one can say that in a two-channel approachaccording to FIG. 29 there is generated by first filtering F_(I) a firstsignal with a first derivative vs. pressure in a predetermined pressurerange Δp and by means of second filtering F_(II) a signal with a secondderivative vs. pressure in the predetermined pressure range Δp, wherebyabsolute values of one of the derivatives is smaller than absolutevalues of the other derivatives in at least one pressure subrange Δp_(s)within the predetermined pressure range Δp. Thereby, the derivatives areconsidered without taking noise into account, i.e. by smoothening therespective output signals, because noise would provide for randomderivatives vs. pressure characteristics.

FIG. 32 shows in a representation in analogy to that of FIGS. 30 and 31the course of signal SF_(I) and at SF_(II) at a predetermined pressurerange to be monitored of about 0 to 195 mbar. Therefrom, it might beseen that first filtering F_(I) in channel K_(I) which accords tofiltering in the frequency area I above transition frequency f_(Tmax) orf_(TΔp) has derivatives vs. pressure which are exclusively positive inpressure subrange Δp_(s1) and exclusively negative in a second pressuresubrange Δp_(s2) out of the overall predetermined pressure range Δp.Further, the derivatives of the second signal S_(FII) are exclusively ofone signum in the overall pressure range Δp.

Further, the absolute values of the derivatives of the first signalS_(FI) are larger at least in the subrange Δp_(s1), thereby again notconsidering noise.

The signal as shown in the FIGS. 30 to 32 are typical for gases andrespective filtering in the two frequency areas above and belowtransition frequency.

The results as shown have been monitored on oxygen gas. With an eye onthe signal courses of FIG. 32 in the pressure range Δp to be monitoredit is evident that the signal S_(FI) has a sensitivity in the pressuresubranges Δp_(s1) and Δp_(s2) which is larger than the sensitivity ofsignal S_(FI) in the respective pressure subranges. On the other handthe signal S_(FI) is ambiguous in that one signal value of S_(FI) may beindicative for two pressure values which are highly different. Forinstance and as shown in FIG. 32 a signal value S_(o) of signal S_(FI)may be indicative for a pressure of about 25 mbar, but also for apressure of about 170 mbar.

Although having a smaller sensitivity, signal S_(FII) is not ambiguousover the range Δp including the subranges Δp_(s1) and Δp_(s2).

Thus, in one embodiment of signal computing as by unit 15 of FIG. 29 thefollowing may be done, according to FIG. 33:

Principally from a signal dependent on the second signal S_(FII) fromchannel K_(II) according to FIG. 29, which accords with filteringresults in frequency area II, the prevailing pressure subrange for theinstantaneously prevailing result of first filtering, S_(FI), isdetermined. As schematically shown in FIG. 33 the prevailing actualsignal S_(FII) is compared with the prestored reference characteristicS_(FIIref) in unit 11 _(II) by comparing unit 13 _(II). The comparisonresult at the non-ambiguous characteristic S_(FIIref) is indicative foran estimate pressure value p_(e) and is fed to a comparator unit 20.There the estimate pressure value p_(e), i.e. the signal indicative forthat value, is compared with limit values for the pressure subrangesΔp_(s), according to Δp_(s1) and Δp_(s2) of FIG. 32. The result of thiscomparison is an output signal which is indicative of the pressuresubrange Δp_(s), namely with an eye on FIG. 32 of either Δp_(s1) orΔp_(s2).

The output signal of comparator unit 20 controls via a control input C₂₁the activation of either a storage unit 23 a or 23 b both being e.g.look-up tables. In the one storage unit 23 a the characteristicS_(FIref) of the output signal of first filtering F_(I) in the firstpressure subrange according to Δp_(s1) as of FIG. 32 is prestored,whereas in storage unit 23 b again e.g. in the form of a look-up table,the characteristic S_(FIref) of the result of first filtering F_(I) inthe pressure subrange Δp_(s2) is prestored.

The pressure subrange indicative signal Δp_(s) controls which of the twoprestored characteristics is compared in a further comparator unit 25with the prevailing result signal S_(FI) of first filtering F_(I). Thisis schematically shown in FIG. 33 by a controlled selection unit unit27.

Thus, after having determined from a signal which depends on secondfiltering F_(II) the prevailing pressure subrange Δp_(s) from a signalwhich depends on the result of first filtering, F_(I) in the subrangeΔp_(s) the pressure indicative signal is established.

Before proceeding to describing further embodiments of the inventionpractically established dimension indications shall be given which havebeen used for oxygen pressure monitoring:

-   -   Center frequency f_(ZII) of second band pass filter F_(II)        relative to modulation frequency f_(C):        10≦f _(ZII) /f _(C)≦20    -   Center frequency f_(ZI) of first band pass filter F_(I):        50≦f _(ZI) /f _(C)≦120    -   Bandwidth of second band pass filter B_(PII):        1≦B _(II) /f _(C)≦18    -   Bandwidth of first band pass filter B_(I):        50≦B _(I) /f _(C)≦1000    -   Wavelength derivation H according to FIG. 1 of laser modulation        at least 5 pm, thereby preferably:        50 pm≦H≦500 pm

Thereby, as a modulatable laser a Vertical Cavity Surface Emitting Laserwas used, modulated at f_(C) ^(˜)800 Hz.

The invention with all different embodiments described up to now is, ina further embodiment, applied in a practical system as will now bedescribed, whereby further embodiments shall be addressed.

In FIG. 34 there is schematically shown a system for monitoring pressureof a gas species in closed containers which are transparent to the laserlight as was described in the modulated wavelength band and which may befilled or not. Such containers, wherein the oxygen pressure has to beaccurately monitored, are e.g. vials of glass or plastic containing afilling material which is not to be exposed to oxygen.

Thus, in a further embodiment of the present invention the gas speciesto be monitored is within a closed and transparent container,transparent to the light of the laser as applied. According to FIG. 34such containers as e.g. glass or plastic material vials 27 are conveyedafter having been filled and sealingly closed and possibly stored duringshorter or longer time spans in atmospheric air, by means of a conveyorarrangement 29 as e.g. a carousel conveyor towards a gas pressure,specifically an oxygen pressure, monitoring station 31. Therein andaccording to FIG. 1 there is provided laser arrangement 1 modulatable aswas described by means of modulating generator 9. The wavelengthmodulated laser beam B enters after having passed a container 27 undertest as transmitted beam B_(Tr) receiver unit 29 with optoelectricconverter 5 according to FIG. 1 and subsequent evaluation unit 7.Depending on the construction of the system the container 27 under testcontinues to be conveyed by conveyor 29 during gas pressure monitoring,whereby in this case laser arrangement 1 and receiver unit 29 are movedalong a predetermined trajectory path in synchronism with the conveyedcontainer 27 under monitoring test.

Otherwise, in another construction, the container 27 under test isstopped so that the laser arrangement 1 and receiver arrangement 29 maybe stationary.

The energy of the transmitted laser beam B_(Tr) may thereby beinfluenced by the prevailing transparency of the overall transmissionpath in fact between laser arrangement 1 and optical input port ofreceiver unit 29, thereby especially by varying transparency of thecontainer, be it due to tolerances of container material, of containerwall contamination etc.

This is taken into account in that there is generically performed atransmission indicative measurement. As transmission influences theenergy of the signals which were described to be filtered as well assignificance of reference characteristics as in look-up tables withwhich actual filtering results are compared, the transmission indicativesignal is applied for weighing such signals.

In spite of the fact that a prevailing transparency in the transmittancepath 3 according to FIG. 1 may be measured as by making use of aseparate laser beam, in one embodiment the laser beam B itself isexploited to provide also for the transparency indicative information.With an eye on FIG. 4 it was explained that the spectrum of signal S₅contains the distinct spectral line at the modulation frequency f_(C).Dependent on how wavelength modulation of the laser beam is realized,the spectral component at f_(C) will be of higher or lower energy. Whenmaking use e.g. of a Vertical Cavity Surface Emitting Laser (VCSEL),where the center wavelength of emission spectrum is tunable and thusmodulatable by amplitude modulating the forward current as is taught inH. P. Zappe et al. “Narrow-linewidth vertical-cavity surface-emittinglasers for oxygen detection”, Appl. Opt. 39 (15), 2475–2479 (May 2000),the energy of the transmitted laser beam at frequency f_(C) is quitelarge. Therefrom results that the energy of the signal S₅ is per seindicative of transmission. As has been explained in all filtering modesfor evaluating the pressure indicative information in signal S₅ theenergy at the frequency f_(C) is not considered by selecting allfiltering with a lowest-most cut-off frequency above the modulationfrequency f_(C).

According to FIG. 35 there is therefore provided a third channel K_(CAL)as a calibration channel, whereat directly or possibly and as shown indashed line via selective band pass filtering at frequency f_(C), atransmission indicative signal S_(Tr) is generated. This transmissionindicative signal is used generically for weighing signals from whichthe pressure indicative signal depends. In the embodiments as weredescribed and according to FIG. 35, whereat the filtering result signalsS_(FI) and S_(FII) are used to find the according pressure-dependentvalue in look-up tables 11 _(I) and 11 _(II), irrespective whichtransition frequency f_(Tmax) or f_(T)Δp has been selected, thetransmission indicative signal S_(Tr) is applied to calibration units 33which are provided at the output of the look-up table units 11 _(I), 11_(II) as well as at the outputs of the filtering units F_(I) and F_(II).

Thus, the output signals which appear at the outputs of the respectivecomparing units 13 become independent from the instantaneouslyprevailing transmission characteristics of the transmission path 3according to FIG. 1 and with an eye on FIG. 34 independent of possiblyvarying transmission of containers 27 which are monitored in line, i.e.in a stream of subsequent containers, with respect to the pressure of aspecific gas species contained therein, as was addressed, especially ofoxygen.

Still a further embodiment comprises to perform the gas species pressuremonitoring as was described up to now upon such gas specimen which has apredetermined known pressure, be it for checking purposes of the overallfunctioning and of accuracy of the system as described, be it forproviding a reference pressure indicative signal on the gas speciesunder test. With an eye on the teaching of FIG. 34 providing for suchstandard monitoring may be done always after a predetermined number ofcontainers 27 have been tested and may even be done before each of thecontainers 27 is monitored on prevailing gas species, in the presentcase on oxygen.

In FIG. 35 there is shown one embodiment for performing a standard orreference monitoring as was just addressed before gas species pressuremonitoring at each of the containers 27.

According to FIG. 36 each container 27 to be tested and once conveyed byconveyor 29 into the monitoring station 31 according to FIG. 34 isgripped by a transfer member 35. The transfer member 35 may comprise atubular member as shown with controlled gripping arrangement 37 and witha laser transition pass through 39. The transfer member 35 is movableperpendicularly to the path of conveyor 29 up and down as shown by thedouble-arrow F, thereby driven in a controlled manner by a drive 41. Thelaser arrangement 1 and the optoelectric converter arrangement 5 arelocated above the conveyor 29, so that whenever a container 27 to betested is gripped in the position as shown, and is then lifted by meansof drive 41 and member 35 in a monitoring position where thepass-through 39 opens the transition path for laser beam B. In FIG. 36the transfer member 35 is shown in its lowered position, where the nextcontainer 27 to be tested is about to be gripped. The transfer member 35further holds a standard or reference container 27 _(ST), wherein apredetermined amount of the gas species to be monitored is present,resulting in a predetermined pressure at a given temperature. Thestandard container 27 _(ST) within transfer member 35 is only rarelyreplaced. Together with transfer member 35 it is moved by means ofcontrolled drive 41 up and down and as a second pass-through 39 _(ST) isprovided in transfer member 35 at a position according to the mountingposition of standard container 27 _(ST), whenever the transfer member 35is in the position as shown in FIG. 36, the laser beam B transitsthrough the standard container 27 _(ST). Thereby, reference monitoringis performed. In dashed line FIG. 36 shows the position of standardcontainer 27 _(ST) whenever container 27 to be monitored is lifted inmonitoring position.

Due to the fact that the pressure of a gas species in the closedcontainers is dependent on the temperature which is substantially equal,as given by the surrounding, for the containers to be tested and forstandard container 27 _(ST) within transfer member 35, the gas speciescontained within standard container 27 _(ST) will be subjected topressure variations due to the same temperature variations as the gasspecies possibly contained in the containers 27 to be tested. Accordingto FIG. 36 after optoelectric conversion of the transmitted laser beam Bsignal processing is performed as was largely explained up to nowfinally at the signal computing unit 15. The signal generated at theoutput A₁₅ of computing unit 15 represents, as was explained, a signalwhich is indicative of the pressure of the specific gas species asmomentarily monitored. As shown in FIG. 36, the standard or referencecontainer 27 _(ST) is also gas pressure monitored, and the result at theoutput A₁₅ is stored in a storing unit 41. Then the subsequentmonitoring test result, which is generated at a container 27 to betested, is fed to a difference forming unit 43 together with the storedreference result value in storing unit 41. Therefore, an output signalD_(p) at the output of difference unit 43 is generated, which isindicative of the difference between the gas pressure indicative signalas monitored at the reference or standard container 27 _(ST) and thenext prevailing conveyed container 27 to be test monitored.

A time control unit 45 controls, as schematically shown in FIG. 35,storing of the output signal of unit 15 in storage unit 41, applying theprevailing output signal of unit 15 together with the stored referencesignal to the difference unit 43 and up/down movement F of transfermember 35 via controlled drive 41.

Further, and as only schematically shown in FIG. 36, the resultingdifference signal D_(p) is fed to a threshold unit 45, where it ischecked whether it reaches a predetermined threshold value or not, aspreestablished by a threshold setting unit 47. Dependent on thecomparison result of D_(p) with the preset threshold value, thecontainer 27 which is momentarily under test will be considered asfulfilling predetermined conditions with respect to the content of thegas species and will then be considered to be a closed, transparentcontainer which holds at most a predetermined maximum amount of gasspecies, especially of oxygen, thus being a regular container. Thosecontainers, which do not fulfill the said conditions and thus have e.g.a too high amount of oxygen, are discarded as shown in FIG. 36schematically with discarding switch S₄₅ controlled from thresholdchecking at threshold unit 45.

As a further embodiment the storage unit 41 is replaced by an averagingunit, whereat subsequent reference pressure indicative signals monitoredat standard containers 27 _(ST) are averaged and the averaged result isfed to the difference unit 43. Thereby, whenever the standard container27 _(ST) is corrupt for whatever reason, its monitoring will notabruptly change the averaging result and thus the prevailing differenceresult D_(p) will still remain accurate for some containers 27, therebynot leading to containers 27 under test being erroneously considered asfulfilling the predetermined gas conditions or not. Thereby (not shown)whenever the monitoring result of a standard container 27 _(ST) leaves apredetermined signal range, an alarm may be established informing aboutcorruption of the standard container 27 _(ST).

Further, FIGS. 34 and 35 show single test station embodiments. Forincreasing throughput of tested containers 27 more than one test station31 may be provided, operating in parallel, so that the test cycle timeis divided by the number of parallel test stations allowing forincreased speed of conveying the containers 27 by means of conveyor 29.

By means of the disclosed technique for gas pressure monitoring, appliedfor testing oxygen content in glass or plastic vials, test cycle timeslower than 0.3 sec. were reached, allowing to accordingly testcontainers in a stream and thereby to test every single container 27.

The invention claimed is:
 1. A method of monitoring pressure of a gasspecies up to at most a predetermined maximum pressure value comprisingexposing said gas species to transmission of laser light; periodicallymodulating the wavelength of said laser light over a wavelength bandincluding at least one absorption line of said gas species;optoelectrically converting said transmitted laser light, therebygenerating an electric output signal; performing at least one of firstfiltering said electric output signal with a filter characteristichaving a lower cut-off frequency not lower than a transition frequencyand of second filtering said electric output signal with a band passfilter characteristic having an upper cut-off frequency not higher thansaid transition frequency and a lower cut-off frequency above modulationfrequency of said periodic wavelength modulation, thereby determiningsaid transition frequency in the spectrum of said electric output signalthere, where the caustic function of the pressure-dependent spectrumenvelopes of said electric signal touches the envelope of said spectrumat said maximum pressure, and evaluating the output of at least one ofsaid filterings as a pressure indicative signal.
 2. The method of claim1, further comprising performing both of said first and secondfilterings.
 3. The method of claim 1, further comprising performing saidfirst filtering as a band pass filtering.
 4. The method of claim 1,further comprising performing said first filtering with a lower cut-offfrequency higher than said transition frequency.
 5. The method of claim1, further comprising performing said first filtering as a band passfiltering and determining a filter frequency above said lower cut-offfrequency of said first filtering there, where the derivatives ofspectral amplitude vs. pressure of said electric output signal at leastapproximately accord with a desired characteristic and performing saidband pass first filtering with said filter frequency as band pass centerfrequency.
 6. The method of claim 1, performing said first filtering asband pass filtering and selecting bandwidth of said band pass firstfiltering with the target of achieving a desired signal to noise ratio.7. The method of claim 1, further comprising performing said firstfiltering as a band pass filtering and realizing a desired sensitivityof said output signal of said band pass first filtering underconsideration of noise by performing the following steps one or morethan one time: a) determining a filter frequency above said lowercut-off frequency of said first filtering there where the derivative ofspectral amplitude vs. pressure of said electric output signal at leastapproximately accords with a desired characteristic and establishing theband pass center frequency of said band pass first filtering at saidfilter frequency; b) tailoring bandwidth of said band pass firstfiltering for a desired signal to noise ratio range.
 8. The method ofclaim 1, further comprising selecting said upper cut-off frequency ofsaid second filtering below said transition frequency.
 9. The method ofclaim 1, further comprising performing said second filtering with acenter frequency there where the derivative of spectral amplitude vs.pressure of said electric output signal at least approximately accordswith a desired characteristic.
 10. The method of claim 1, furthercomprising performing said second filtering with a bandwidth for adesired signal to noise ratio.
 11. The method of claim 1, furthercomprising realizing a desired sensitivity of said output signal of saidsecond filtering under consideration of noise by performing thefollowing steps one or more than one times: a) determining a centerfrequency of said second filtering there where the derivative ofspectral amplitude vs. pressure of said electric output signal at leastapproximately accords with a desired characteristic and b) tailoringbandwidth of said second filtering for a desired signal to noise ratio.12. A method of monitoring pressure of a gas species within apredetermined pressure range between a maximum pressure value and aminimum pressure value comprising exposing said gas species totransmission of laser light; periodically modulating the wavelength ofsaid laser light over a wavelength band including at least oneabsorption line of said gas species; optoelectrically converting saidtransmitted laser light, thereby generating an electric output signal;performing at least one of first filtering said electric output signalwith a filter characteristic having a lower cut-off frequency not lowerthan a transition frequency and of second filtering said electric outputsignal with a band pass filter characteristic having an upper cut-offfrequency not higher than said transition frequency and a lower cut-offfrequency above modulation frequency of said periodic wavelengthmodulation, thereby determining said transition frequency in thespectrum of said electric output signal there where the spectrumenvelopes of said electric output signal at said minimum and at saidmaximum pressure values intersect, and evaluating the output of at leastone of said filterings as a pressure indicative signal.
 13. The methodof claim 12, further comprising performing both of said first and secondfilterings.
 14. The method of claim 12, further comprising performingsaid first filtering as a band pass filtering.
 15. The method of claim12, further comprising performing said first filtering with a lowercut-off frequency higher than said transition frequency.
 16. The methodof claim 12, further comprising performing said first filtering as bandpass filtering between said transition frequency and a noise limitfrequency, said noise limit frequency being defined there where noiseenergy of said electric output signal equals signal energy of saidelectric output signal at said minimum pressure value.
 17. The method ofclaim 16, further comprising selecting said band pass first filtering sothat the energy difference in the spectrum of said electric outputsignal between applying said maximum and said minimum pressure becomesmaximum.
 18. The method according to claim 17, thereby selecting saidfirst band pass filtering under the constraint that noise energy of saidelectric output signal, where said filtering is effective, is at mostequal to signal energy at said maximum pressure.
 19. The method of claim12, further comprising selecting said upper cut-off frequency of saidsecond filtering below said transition frequency.
 20. The method ofclaim 12, further comprising performing said second filtering therewhere the energy difference in the spectrum of said electric outputsignal between applying said maximum and applying said minimum pressureis maximum.
 21. The method of claim 1, wherein said gas species isoxygen.
 22. The method of claim 1, further comprising generating areference pressure indicative signal by performing said monitoring at apredetermined pressure of said gas species.
 23. The method of claim 22,further comprising generating a resulting pressure indicative signal independency of a difference of said reference pressure indicative signaland said pressure indicative signa.
 24. The method of claim 1, furthercomprising monitoring transmission of said laser light along atrajectory path to which said gas species is or is to be applied,generating thereby a transmission indicative signal and weightingsignals in dependency of said transmission indicative signal, from whichsaid pressure indicative signal depends.
 25. The method of claim 1,further comprising providing a gas with said gas species in a containertransparent for said laser light.
 26. The method claim 25, furthercomprising providing said container in ambient air.
 27. The method ofclaim 25, further comprising monitoring transparency of a trajectorypath for said laser light including said transparent container,generating thereby a transparency indicative signal and weightingsignals in dependency of said transmission indicative signal, from whichsaid pressure indicative signal depends.
 28. The method of claim 27,further comprising providing a third parallel channel as a calibrationchannel and generating in said third channel said transmissionindicative signal.
 29. The method of claim 25, further comprisinggenerating a reference signal by performing said monitoring at areference container with a predetermined amount pressure of said gasspecies thereby generating a result pressure indicative signal independency of a difference from said reference signal and said pressureindicative signal.
 30. The method of claim 1, further comprisingchecking said pressure indicative signal on plausibility.
 31. The methodof claim 25, further comprising monitoring oxygen pressure.
 32. Themethod of claim 25, wherein said container is filled with a product. 33.The method of claim 25, wherein said container is substantially of glassor plastic material.
 34. The method of claim 33, wherein said containeris a vial.
 35. The method of claim 25, wherein said container is one ofa multitude of containers conveyed in a stream towards and from saidmonitoring. during said monitoring.
 36. The method of claim 35, furthercomprising moving said container and laser light transmitted throughsaid container in synchronism during said monitoring.
 37. The method ofclaim 25, further comprising monitoring subsequently said gas pressurein a multitude of said containers conveyed subsequently towards and fromsaid monitoring and generating a reference pressure indicative signal byapplying to said monitoring at least one reference container with apredetermined amount of said gas species before monitoring one of saidmultitude of containers and generating a result pressure indicativesignal in dependency of said reference pressure indicative signal andsaid pressure indicative signal.
 38. The method of claim 37, furthercomprising generating said reference pressure indicative signal beforemonitoring each of said multitude of containers.
 39. The method of claim37, further comprising averaging subsequent of said reference pressureindicative signals and forming said difference in dependency of a resultof said averaging.
 40. A method for manufacturing closed, filledcontainers transparent to laser light and with a predetermined maximumamount of oxygen comprising manufacturing closed, filled, transparentcontainers, subjecting said containers to gas pressure monitoringaccording to one of the claims 1 to 39 and rejecting containers if thepressure indicative signal is indicative for an oxygen pressure in acontainer above a predetermined maximum value.
 41. A method formonitoring pressure of a gas species comprising exposing said gasspecies to transmission of laser light; periodically modulating thewavelength of said laser light over a wavelength band including at leastone absorption line of said gas species; optoelectrically convertingsaid transmitted laser light, thereby generating an electric outputsignal; inputting a signal dependent on said electric output signal toat least a first and a second, parallel, gas pressure monitoringchannel; performing in said first channel first filtering; performing insaid second channel second filtering; performing said first filtering sothat the output signal of said first filtering varies with a firstcharacteristic as a function of said pressure; performing said secondfiltering so that the output signal of said second filtering varies witha second characteristic as a function of said pressure; said firstcharacteristic being different from said second characteristic;evaluating from combining signals dependent on the output signals ofsaid first and second filtering said pressure indicative signal.
 42. Themethod of claim 41, further comprising at least one of said first and ofsaid second filtering as band pass filtering.
 43. The method of claim41, further comprising performing said first and second filtering innon-overlapping frequency areas of the spectrum of said electric outputsignal.
 44. The method of claim 41, further comprising performing saidfirst and said second filtering as band pass filtering.
 45. The methodof claim 41, further comprising performing said first and secondfiltering in first and second frequency ranges respectively, the energyof said electric output signal having a first energy vs. pressurecharacteristic in said first frequency range and a second energy vs.pressure characteristic in said second frequency range, said first andsecond energy characteristics being different from each others.
 46. Themethod of claim 45, further comprising performing at least one of saidfirst and second filtering as band pass filtering.
 47. The method ofclaim 41, further comprising prestoring a first reference characteristicrepresenting said first characteristic and prestoring a second referencecharacteristic representing said second characteristic and comparingsignals dependent from output signals of said first and second filteringrespectively with said first and second reference characteristics,thereby generating first and second pressure indicative signals.
 48. Themethod of claim 41, said first filtering generating a first outputsignal with first derivatives vs. pressure in a predetermined pressurerange, said second filtering generating a second output signal withsecond derivatives vs. pressure in said predetermined pressure range,absolute values of one of said derivatives being smaller than absolutevalues of the other of said derivatives in at least one commonsub-pressure range of said predetermined pressure range, consideringsaid derivatives without noise.
 49. The method of claim 41, said secondfiltering generating a second output signal with exclusively positive ornegative second derivatives vs. pressure in a predetermined pressurerange, said first filtering generating a first output signal with firstderivatives vs. pressure being exclusively positive in at least onepressure subrange of said predetermined pressure range and beingexclusively negative in at least one second pressure subrange of saidpredetermined range, the absolute values of said second derivativesbeing smaller in at least one of said subranges than absolute values ofsaid first derivatives in said at least one of said subranges andconsidering derivatives without noise.
 50. The method of claim 49,further comprising determining from a signal dependent on said secondoutput signal one of said pressure subranges.
 51. The method of claim50, further determining from a signal dependent on said first outputsignal in said determined pressure subrange a pressure indicativesignal.
 52. The method of claim 41, further comprising predetermining amaximum pressure to be monitored; performing at least one of performingsaid second filtering with a band pass filter characteristic having anupper cut-off frequency not higher than a transition frequency and alower cut-off frequency above modulation frequency of said periodicwavelength modulation and of performing said first filtering with afilter characteristic having a lower cut-off frequency not lower thansaid transition frequency; thereby determining said transition frequencyin the spectrum of said electric output signal there where the causticfunction of the pressure-dependent spectrum envelopes of said electricoutput signal touches the envelope of said spectrum at said maximumpressure.
 53. The method of claim 52, further comprising performing saidfirst and second filterings.
 54. The method of claim 52, furthercomprising performing said first filtering as a band pass filtering. 55.The method of claim 52, further comprising performing said firstfiltering with a lower cut-off frequency higher than said transitionfrequency.
 56. The method of claim 52, further comprising performingsaid first filtering as a band pass filtering and determining a filterfrequency above said lower cut-off frequency of said first filteringthere where the derivatives of spectral amplitude vs. pressure of saidelectric output signal at least approximately accord with a desiredcharacteristic and performing said band pass first filtering with saidfilter frequency as band pass center frequency.
 57. The method of claim52, further comprising performing said first filtering as band passfiltering and selecting bandwidth of said band pass first filtering withthe target of achieving a desired signal to noise ratio.
 58. The methodof claim 52, further comprising performing said first filtering as aband pass filtering and realizing a desired sensitivity of output signalof said band pass first filtering under consideration of noise byperforming the following steps one or more than one time: a) determininga filter frequency above said lower cut-off frequency of said firstfiltering there where the derivative of spectral amplitude vs. pressureof said electric output signal at least approximately accords with adesired characteristic and establishing the band pass center frequencyof said band pass first filtering at said filter frequency; b) tailoringbandwidth of said band pass first filtering for a desired signal tonoise ratio.
 59. The method of claim 52, further comprising selecting anupper cut-off frequency of said second filtering below said transitionfrequency.
 60. The method of claim 52, further comprising performingsaid second filtering with a center frequency there where the derivativeof spectral amplitude vs. pressure of said electric output signal atleast approximately accords with a desired characteristic.
 61. Themethod of claim 52, further comprising performing said second filteringwith a bandwidth for a desired signal to noise ratio.
 62. The method ofclaim 52, further comprising performing said second filtering andrealizing a desired sensitivity of an output signal of said secondfiltering under consideration of noise by performing the following stepsone or more than one time: a) determining a center frequency of saidsecond filtering there where the derivative of amplitude vs. pressure ofsaid electric output signal at least approximately accords with adesired characteristic and b) tailoring bandwidth of said secondfiltering for a desired signal to noise ratio.
 63. The method of claim41, further comprising performing said monitoring in a predeterminedpressure range between a minimum pressure and a maximum pressure;performing at least one of said second filtering with a band pass filtercharacteristic having an upper cut-off frequency not higher than atransition frequency and a lower cut-off frequency above modulationfrequency of said periodic wavelength modulation, and of said firstfiltering with a filter characteristic having a lower cut-off frequencynot lower than said transition frequency; thereby determining saidtransition frequency in the spectrum of said electric output signalthere where the spectrum envelopes of said electric output signal atsaid minimum and at said maximum pressure values intersect.
 64. Themethod of claim 63, further comprising performing said first and secondfiltering.
 65. The method of claim 63, further comprising performingsaid first filtering as a band pass filtering.
 66. The method of claim63, further comprising performing said first filtering with a lowercut-off frequency higher than said transition frequency.
 67. The methodof claim 63, further comprising performing said first filtering as bandpass filtering between said transition frequency and a noise limitfrequency, said noise limit frequency being defined there where noiseenergy of said electric output signal equals signal energy of saidelectric output signal at said minimum pressure value.
 68. The method ofclaim 67, further comprising selecting said band pass first filtering sothat the energy difference in the spectrum of said electric outputsignal between applying said maximum and applying said minimum pressurebecomes maximum.
 69. The method according to claim 68, thereby selectingsaid band pass first filtering under the constraint that noise energy ofsaid electric output signal, where said filtering is effective, is atmost equal to signal energy at said maximum pressure.
 70. The method ofclaim 63, further comprising selecting said upper cut-off frequency ofsaid second filtering below said transition frequency.
 71. The method ofclaim 63, further comprising performing said second filtering therewhere the energy difference in the spectrum of said electric outputsignal between applying said maximum and applying said minimum pressuresis maximum.
 72. The method of claim 63, further comprising selecting theupper cut-off frequency of said first filtering below said transitionfrequency.